Retroviral vectors have been constructed that express one or more of the following gene products: sCD4 (or it's derivatives), transdominant rev mutants, antisense TAR RNA, and an HIV inducible alpha-interferon or diphtheria toxin. The goal of this work is to engineer human hematopoietic cells such that they manifest anti-HIV activity and thus may prevent HIV infection or decrease viral replication. Initial work has concentrated on construct retroviral vectors that produce a secreted form of the helper/inducer T-cell antigen, CD4 (sCD4). Analysis of this system has been carried out through the in vivo animal testing stage. We have demonstrated that sCD4 transduced primary human lymphocytes can be directly protected from HIV infection in vitro. We have further documented protection of human lymphocytes from infection by HIV in the Hu-PBL-SCID mouse AIDS animal model. As part of our efforts to develop retroviral vectors that can be specifically targeted to CD4 or HIV infected cells, we have been studying the basic functional properties of the retroviral vector envelope. By substituting a glycosyl-phosphotidylinositol (GPI) membrane anchor for part or all of the retroviral envelope transmembrane protein (TM) and creating several deletion variants of the TM subunit we have begun to dissect its role in envelope function. We have extended our work on fusion by showing that a genetically engineered MuLV envelope that encodes a TM subunit identical to one found in virions (p12E) mediates cell membrane fusion at neutral pH in both transformed and untransformed cell lines. We propose that it is the processing of the R peptide from the envelope TM (p15E) at the time of virus budding or within virions that renders the envelope competent to fuse.